docs(spec): describe() for SpecialOperator $null and $exists

.changeset/spec-special-operator-describe.md

@@ -0,0 +1,29 @@
+---
+'@objectstack/spec': patch
+---
+
+docs(spec): `SpecialOperator` gains the `.describe()` that puts `$null` / `$exists` on the reference page (#14048)
+
+`SpecialOperatorSchema` declared both members as bare `z.boolean().optional()` with a
+JSDoc comment and no `.describe()`. `content/docs/references/data/filter.mdx` fills its
+Description column from `prop.description` — the JSON-Schema projection of a Zod
+`.describe()` — so both cells rendered **empty**, and the published reference page said
+nothing whatsoever about the two operators whose meaning was the subject of a six-site
+correction campaign (#13539, #13709). A reader could not learn from that page that
+`$exists` asks whether the field HAS A VALUE.
+
+Each member now carries a `.describe()`; the JSDoc stays as the source of truth it
+already was and the describe restates it. The regenerated page gains exactly two
+Description cells (`filter.mdx:138-139`) and nothing else.
+
+Prose only. No accept/reject or shape change: both members were and stay
+`z.boolean().optional()`, so the only JSON-Schema delta is a `description` string on two
+properties. Verified on a freshly built `dist`: `check:generated` reported
+`content/docs/references/**` as the single stale artifact and `--fix` regenerated only
+`filter.mdx`; `check:api-surface`, `check:export-origins` and `check:authorable-surface`
+stayed green with no new export.
+
+The wording was written against `scripts/check-corpus-claim-drift.mjs` rather than into
+it: that shrink-only ratchet watches `content/docs/**` for `$exists` prose, its
+`exists-key-presence` row reads `exists-key-presence 4` both before this diff and after
+the page was regenerated, so the new prose adds zero claim sites.

content/docs/references/data/filter.mdx

@@ -135,8 +135,8 @@ Type: `[FilterArray](#filterarray)[]`
 
 | Property | Type | Required | Description |
 | :--- | :--- | :--- | :--- |
-| **$null** | `boolean` | optional |  |
-| **$exists** | `boolean` | optional |  |
+| **$null** | `boolean` | optional | Is-null check. `true` matches rows where the field is null, `false` matches rows where it is not null. Lowered to `IS NULL` (true) / `IS NOT NULL` (false) on the SQL family and to `{ field: null }` (true) / `{ $ne: null }` (false) on MongoDB. |
+| **$exists** | `boolean` | optional | Has-a-value check — the exact inverse of `$null`. `true` matches rows where the field holds a value (`!= null`), `false` matches rows where it holds none. Portable across every backend the platform ships: lowered to `IS NOT NULL` (true) / `IS NULL` (false) on the SQL family and to `{ $ne: null }` (true) / `{ $eq: null }` (false) on MongoDB. |
 
 
 ---

packages/spec/src/data/filter.zod.ts

@@ -1025,14 +1025,24 @@ export function likePatternToGlobPattern(pattern: string): string {
  */
 export const SpecialOperatorSchema = lazySchema(() => z.object({
   /** Is null check - SQL: IS NULL (true) / IS NOT NULL (false) | MongoDB: field: null */
-  $null: z.boolean().optional(),
+  $null: z.boolean().optional().describe(
+    'Is-null check. `true` matches rows where the field is null, `false` matches rows '
+    + 'where it is not null. Lowered to `IS NULL` (true) / `IS NOT NULL` (false) on the '
+    + 'SQL family and to `{ field: null }` (true) / `{ $ne: null }` (false) on MongoDB.'
+  ),
 
   /**
    * Field HAS A VALUE (`!= null`) — the inverse of `$null`, never key presence.
    * Lowered to `IS NOT NULL` (true) / `IS NULL` (false) on SQL and to
    * `{$ne: null}` / `{$eq: null}` on MongoDB.
    */
-  $exists: z.boolean().optional(),
+  $exists: z.boolean().optional().describe(
+    'Has-a-value check — the exact inverse of `$null`. `true` matches rows where the '
+    + 'field holds a value (`!= null`), `false` matches rows where it holds none. '
+    + 'Portable across every backend the platform ships: lowered to `IS NOT NULL` (true) / '
+    + '`IS NULL` (false) on the SQL family and to `{ $ne: null }` (true) / '
+    + '`{ $eq: null }` (false) on MongoDB.'
+  ),
 }));
 
 // ============================================================================